1. Field of the Invention
The present invention generally relates to an apparatus and method for completing a wellbore. More particularly, the invention relates to an apparatus and method for expanding a tubular body in a wellbore.
2. Description of the Related Art
In well completion operations, a wellbore is formed to access hydrocarbon-bearing formations by the use of drilling. Drilling is accomplished by utilizing a drill bit that is mounted on the end of a drill support member, commonly known as a drill string. To drill within the wellbore to a predetermined depth, the drill string is often rotated by a top drive or rotary table on a surface platform or rig, or by a downhole motor mounted towards the lower end of the drill string. After drilling to a predetermined depth, the drill string and drill bit are removed and a section of casing is lowered into the wellbore. An annular area is thus formed between the string of casing and the formation. The casing string is temporarily hung from the surface of the well. A cementing operation is then conducted in order to fill the annular area with cement. Using an apparatus known in the art, the casing string is cemented into the wellbore by circulating cement into the annular area defined between the outer wall of the casing and the borehole. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, the well is drilled to a first designated depth with a drill bit on a drill string. The drill string is removed. A first string of casing or conductor pipe is then run into the wellbore and set in the drilled out portion of the wellbore, and cement is circulated into the annulus behind the casing string. Next, the well is drilled to a second designated depth, and a second string of casing, or liner, is run into the drilled out portion of the wellbore. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string is then fixed, or “hung” off of the existing casing by the use of slips which utilize slip members and cones to wedgingly fix the new string of liner in the wellbore. The second casing string is then cemented. This process is typically repeated with additional casing strings until the well has been drilled to total depth. As more casing strings are set in the wellbore, the casing strings become progressively smaller in diameter in order to fit within the previous casing string. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
Decreasing the diameter of the wellbore produces undesirable consequences. Progressively decreasing the diameter of the casing strings with increasing depth within the wellbore limits the size of wellbore tools which are capable of being run into the wellbore. Furthermore, restricting the inner diameter of the casing strings limits the volume of hydrocarbon production fluids which may flow to the surface from the formation.
In the last several years, methods and apparatus for expanding the diameter of casing strings within a wellbore have become feasible. For example, a string of liner can be hung in a well by placing the upper portion of a second string of casing in an overlapping arrangement with the lower portion of a first string of casing. The second string of casing is then expanded into contact with the existing first string of casing with an expander tool. The second string of casing is then cemented.
An exemplary expander tool utilized to expand the second casing string into the first casing string is fluid powered and run into the wellbore on a working string. The hydraulic expander tool includes radially expandable members which, through fluid pressure, are urged outward radially from the body of the expander tool and into contact with the second casing string therearound. As sufficient pressure is generated on a piston surface behind these expansion members, the second casing string being acted upon by the expansion tool is expanded past its point of elastic deformation. In this manner, the inner and outer diameter of the expandable tubular is increased in the wellbore. By rotating the expander tool in the wellbore and/or moving the expander tool axially in the wellbore with the expansion member actuated, a tubular can be expanded into plastic deformation along a predetermined length in a wellbore.
Recently, an expansion system has been developed to line a borehole with an entire section of expandable tubing. Generally, the expansion system 65 includes a liner assembly 75 and an expansion assembly 85 as will discussed in prior art FIGS. 1A–1F. Prior to running the expansion system 65 into the wellbore, a borehole 50 is formed below an existing string of casing 60 by a standard drill bit (not shown). To prepare the borehole 50 for placement of the expansion system 65, an under-reaming procedure is employed using a standard under-reamer 55 to enlarge the inside diameter of the borehole 50 as illustrated in FIG. 1A. Thereafter, the expansion system 65 is run into the under-reamed borehole 50 as shown in FIG. 1B. The liner assembly 75 includes a string of expandable liner 70 with a preformed launcher section 30 formed at the lower end thereof. The expansion assembly 85 includes an expander cone 35 that is placed in the preformed launcher section 30 prior to running the expansion system 65 into the under-reamed borehole 50. After the placement of the expansion system 65, cement is pumped through the expansion system 65 to fill an annulus 40 formed between the expansion system 65 and the surrounding borehole 50 as shown in FIG. 1C. Prior to the curing of the cement, fluid is pumped through the expansion system 65 to urge the expander cone 35 through the expandable liner 70 as depicted in FIG. 1D. Subsequently, the expander cone 35 expands an upper portion of the liner 70 into contact with the inside diameter of the casing 60 to form a sealing relationship therebetween as shown in FIG. 1E. Next, the expansion assembly 85 is then removed from the borehole 50 and a mill 45 is employed to mill out a shoe 80 at the lower end of the liner assembly 75 as illustrated in FIG. 1F.
There are certain disadvantages of using the prior art expansion system illustrated in FIGS. 1A–1F. One disadvantage relates to preparation of the borehole below the existing casing string prior to the placement of the expansion system in the wellbore. More specifically, an under-reaming operation must be conducted after the borehole has been formed in order to enlarge the inner diameter of the borehole so that the expansion system with the preformed launcher portion may be positioned in the borehole. Another disadvantage relates to the fact that a tubular can only be expanded about 22–25% past its elastic limit using the method described above. Expansion past about 22–25% of its original diameter may cause the liner to fracture due to stress. Securing the liner in the borehole by expansion alone would require an increase in diameter of over 25%. Therefore, the cementation operation must be employed to fill in the annulus formed between the expanded liner and the borehole.
There is, therefore, a need for a method and an apparatus for placing a liner in a borehole without preparing the borehole with an under-reaming operation. There is a further need for a method and apparatus for expanding the diameter of a tubular string past the current limit of 25%. There is yet a further need for a method and an apparatus for expanding a lower portion of a casing string or tubular body to a diameter larger than the diameter of the tubular thereabove without compromising the structural integrity.